Communication system

ABSTRACT

A relay communications system is described in which a base station receives a request from an MME that an overload procedure be started or stopped. The base station inserts the identity of the MME into the request and forwards the modified request to a relay node. The relay node then initiates or terminates the overload procedure in accordance with the request for mobile telephones which the MME that made the request serves, but not for mobile telephones which the MME does not serve.

TECHNICAL FIELD

The present invention relates to a communication system and tocomponents thereof for providing relay services to mobile or fixedcommunication devices. The invention has particular but not exclusiverelevance to relay services used in LTE-Advanced as currently defined in3GPP standards documentation TS 36.413 V 9.3.0 and TS 36-300 V 10.0.0.

BACKGROUND ART

Relaying is considered for LTE-Advanced as a tool to improve, forexample, the coverage of high data rates for User Equipment (UE), groupmobility, temporary network deployment, the cell edge throughput and/orto provide coverage in new cell areas. E-UTRAN supports relaying byhaving a Relay Node (RN) wirelessly connected to a base station (eNB)(referred to as a Donor eNB (DeNB)). In addition to serving its own‘donor’ cell, the Donor eNB serves the RN, via a modified version of theE-UTRA radio interface. The modified interface is referred to as the‘Un’ interface.

Each RN is provided with many aspects of a base station's functionalityand is therefore capable of acting as a base station serving its own‘relay’ cell. From the perspective of the user equipment in the relaycell, therefore, the RN essentially appears to be a conventional LTEbase station. In addition to the base station functionality, however,the RN also supports a subset of the UE functionality including, forexample, many aspects of the physical layer, layer-2, radio resourcecontrol (RRC), and non access stratum (NAS) functionality, to allow itto connect wirelessly to a Donor eNB.

Several mobility management entities (MMEs) are located in the core ofthe E-UTRA network. Each MME serves a respective subset of the userequipment in the ‘donor’ cell of the Donor eNB, the relay cell, and/orother cells in the wider network. The Donor eNB forwards controlmessages received from the MME to the RN over the Un interface and, fromthe perspective of the RN, the Donor eNB appears to be a single MME thatserves all the items of user equipment registered with the RN in therelay cell.

Accordingly, the Donor eNB is capable of handling communication both‘directly’ to and from user equipment camped in its own cell and‘indirectly’, via the RN, to and from user equipment camped in the relaycell.

The MMEs in the core network can become overloaded and, accordingly,each MME is provided with an overload handling capability. Specifically,when one of the MMEs experiences an overload situation, it randomlyselects a subset of the base stations that it communicates with, andsends the selected base station(s) a control message to requestinitiation of a procedure to alleviate the overload situation.Typically, for example, the requested overload procedure will involvethe base station rejecting attempts to establish a connection made byuser equipment served by the MME making the request.

SUMMARY OF THE INVENTION

The inventors have, however, realised that there is a problem with thisarrangement. In particular, when a Donor eNB receives an overloadinitiation message from an MME it initiates the overload procedure foruser equipment in its own donor cell. The Donor eNB also forwards theoverload initiation message to the RN. However, because the Donor eNBappears, to the RN, to be a single MME that serves all the userequipment in the relay cell, the relay node responds to the overloadmessage by initiating the overload procedure for all the user equipmentcamped in its relay cell. Thus, in addition to rejecting attempts toestablish a connection made by user equipment served by the overloadedMME, the relay node will also reject connection attempts made by userequipment served by other MMEs regardless of whether the other MMEs areoverloaded or not. Thus, attempts by user equipment in the relay cell toestablish a connection via the RN may be rejected unnecessarily. Thiscan result in an undesirably high incidence of connection rejections inthe relay cell.

The present invention aims to provide an improved communication systemand improved components of the communication system which overcome or atleast alleviate the above issues.

Accordingly one aspect of the present invention provides a relay nodefor a communication system, the relay node comprising: means forassociating a plurality of mobile devices with the relay node; means forreceiving, from a base station, a request that an overload action beperformed and identity information representing an identity of a networkentity that requested the overload action; and means for selectivelyperforming said requested overload action in respect of the mobiledevices associated with the relay node in dependence on whether or notthe mobile devices are served by the network entity having the identityrepresented by the identity information.

The requested overload action may comprise initiating an overloadprocedure. The request for initiating an overload procedure may compriseoverload start message (e.g. an S1 AP: Overload Start Message).

The network entity may comprise a mobility management entity (MME).

The overload procedure may comprise receiving a request to establish aconnection from a mobile telephone associated with the relay node, maycomprise determining whether or not the mobile telephone that made therequest is served by the network entity having the identity representedby the identity information, may comprise generating a message forrejecting said request to establish a connection or generating a messagefor accepting said request to establish a connection in dependence onwhether or not the mobile telephone that made the request is served bythe network entity having the identity represented by the identityinformation, and/or may comprise transmitting said generated message tothe mobile telephone that made the request to establish a connection.

The overload procedure initiated by the performing means may furthercomprise determining a reason for the requested connection establishmentrequest, and wherein said generation step may further comprisegenerating said message for rejecting said request to establish aconnection or generating said message for accepting said request toestablish a connection in dependence on said determined reason for therequested connection.

The requested overload action may comprise terminating an overloadprocedure. The request for terminating an overload procedure maycomprise overload stop message (e.g. an S1 AP: Overload Stop Message)

Another aspect of the present invention provides a base station for acommunication system, the base station comprising: means for receiving,from a network entity, a request that an overload action be performed;means for generating identity information representing an identity ofthe network entity that requested the overload action; and means fortransmitting, to a relay node, the request that the overload action beperformed and the information representing an identity of the networkentity that requested the overload action.

The base station may further comprise means for inserting saidinformation representing an identity of the network entity thatrequested the overload action into said request that the overload actionbe performed, whereby said information representing an identity of thenetwork entity that requested the overload action may be transmitted aspart of said request that the overload action be performed.

The request that the overload action be performed may comprise a requestfor an overload procedure to be initiated (e.g. an Overload Startmessage).

The request that the overload action be performed may comprise a requestfor an overload procedure to be terminated (e.g. an Overload Stopmessage).

According to another aspect of the invention there is provided a networkentity for a communication system, the network entity comprising: meansfor determining that an overload situation has occurred; means forgenerating, in response to determining that an overload situation hasoccurred, a request that an overload action be performed, said requestcomprising information representing an identity of the network entity(e.g. in an information element such as an MME ID IE); and means fortransmitting, to a base station, the request that the overload action beperformed.

According to another aspect of the invention there is provided a methodperformed by a relay node in a communication system, the methodcomprising: associating a plurality of mobile devices with the relaynode; receiving, from a base station, a request that an overload actionbe performed and identity information representing an identity of anetwork entity that requested the overload action; and selectivelyperforming said requested overload action in respect of the mobiledevices associated with the relay node in dependence on whether or notthe mobile devices are served by the network entity having the identityrepresented by the identity information.

According to another aspect of the invention there is provided a methodperformed by a base station in a communication system, the methodcomprising: receiving, from a network entity, a request that an overloadaction be performed; generating identity information representing anidentity of the network entity that requested the overload action; andtransmitting, to the relay node, the request that the overload action beperformed and the information representing an identity of the networkentity that requested the overload action.

According to another aspect of the invention there is provided a methodperformed by a network entity for a communication system, the methodcomprising: determining that an overload situation has occurred;generating, in response to determining that an overload situation hasoccurred, a request that an overload action be performed, said requestcomprising information representing an identity of the network entity;and transmitting, to a base station, the request that the overloadaction be performed.

According to another aspect of the invention there is provided acommunications device for a communication system that comprises aplurality of network entities, a base station, a relay node, and aplurality of mobile devices associated with the relay node, thecommunications device comprising: means for receiving a request that anoverload action be performed for at least one of said plurality ofnetwork entities; and means for selectively performing said requestedoverload action in respect of the mobile devices associated with therelay node in dependence on whether or not the mobile devices are servedby the network entity for which the overload action has been requested.

The communications device may be said relay node or may be said basestation.

The requested overload action may comprise initiating an overloadprocedure, and said overload procedure may comprise receiving a requestto establish a connection from a mobile telephone associated with therelay node, may comprise determining whether or not the mobile telephonethat made the request is served by the network entity for which theoverload action was requested, may comprise generating a message forrejecting said request to establish a connection or generating a messagefor accepting said request to establish a connection in dependence onwhether or not the mobile telephone that made the request is served bythe network entity for which the overload action was requested, and/ormay comprise transmitting said generated message to the mobile telephonethat made the request to establish a connection.

The requested overload action may comprise initiating an overloadprocedure, and said overload procedure may comprises receiving anindication that a connection has been established between a mobiletelephone associated with the relay node and the relay node, maycomprise determining whether or not the mobile telephone for which theconnection is established is served by the network entity for which theoverload action has been requested, may comprise selectively generatinga message for terminating said established connection in dependence onwhether or not the mobile telephone for which the connection isestablished is served by the network entity for which the overloadaction has been requested, and/or may comprise transmitting any messageso generated to the relay node.

The message for terminating said established connection may comprise anerror indication and/or may comprise a message for releasing a contextof the mobile telephone. A message for releasing a context may comprisea LIE Context Release.

According to another aspect of the invention there is provided a methodperformed by a communications device in a communication system thatcomprises a plurality of network entities, a base station, a relay node,and a plurality of mobile devices associated with the relay node, themethod comprising: receiving a request that an overload action beperformed for at least one of said plurality of network entities; andselectively performing said requested overload action in respect of themobile devices associated with the relay node in dependence on whetheror not the mobile devices are served by the network entity for which theoverload action is requested.

According to another aspect of the invention there is provided acomputer program product comprising computer implementable instructionsfor causing a programmable computer device to become configured as therelay node, as the base station, as the network entity, or as thecommunications device.

A preferable embodiment of the invention extends to methods and/orapparatus substantially as herein described with reference to theaccompanying drawings.

Any feature in one aspect of the invention may be applied to otheraspects of the invention, in any appropriate combination. In particular,method aspects may be applied to apparatus aspects, and vice versa.

Furthermore, features implemented in hardware may generally beimplemented in software, and vice versa. Any reference to software andhardware features herein should be construed accordingly.

The invention also provides corresponding methods and computer softwareproducts that may be provided on a carrier signal or on a recordingmedium.

The text of the abstract filed herewith is repeated here as part of thespecification. In an exemplary aspect of the invention of the inventionthere is provided a relay communications system in which a base stationreceives a request from an MME that an overload procedure be started orstopped. The base station inserts the identity of the MME into therequest and forwards the modified request to a relay node. The relaynode then initiates or terminates the overload procedure in accordancewith the request for mobile telephones which the MME that made therequest serves, but not for mobile telephones which the MME does notserve.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the attached figures in which:

FIG. 1 schematically illustrates a mobile telecommunication system of atype to which the described embodiments are applicable;

FIG. 2 is a block diagram illustrating the main components of the basestation illustrated in FIG. 1;

FIG. 3 is a block diagram illustrating the main components of the relaystation illustrated in FIG. 1;

FIG. 4 is a flow diagram illustrating, in general terms, a procedure forinitiating a response to an overload situation performed by componentsof the mobile telecommunication system of FIG. 1;

FIG. 5 is a flow diagram illustrating, in general terms, a procedure forterminating a response to an overload situation performed by componentsof the mobile telecommunication system of FIG. 1;

FIG. 6 is a flow diagram illustrating, in general terms, an overloadprocedure performed by components of the mobile telecommunication systemof FIG. 1; and

FIG. 7 is a simplified overview of a mobile telecommunication system ofa type to which the described embodiments are applicable.

MODE OF CARRYING OUT THE INVENTION Overview

FIG. 1 schematically illustrates a mobile (cellular) telecommunicationsystem 1 including user equipment 3, comprising a plurality of mobiletelephones 3-1 to 3-5 served by a donor base station 5-1, and userequipment 4 comprising a plurality of other mobile telephones 4-1 to 4-3served by another base station 5-2. The telecommunication system 1 alsocomprises a relay node 7 that connects wirelessly to the donor basestation 5-1, and a core network 8 including, amongst other things, aplurality of mobility management entities (MMEs) 9-1, 9-2, 9-3(generally referenced as 9).

Each MME 9 serves a different subset of the mobile telephones 3, 4, viaa respective base station 5-1, 5-2. In FIG. 1, for example, mobiletelephones 3-3, 3-4 and 4-3 are served by MME 9-1, mobile telephones3-1, 3-5 and 4-1 are served by MME 9-2, and mobile telephones 3-2 and4-2 are served by MME 9-3. Each MME 9 may also serve other mobiletelephones via other base stations that are not shown in FIG. 1.

Mobile telephones 3-1, 3-2, and 4 (hereafter referred to as Direct MTs)are each registered directly with a respective base station 5-1, 5-2.Mobile telephones 3-3, 3-4 and 3-5 (hereafter referred to as Relay MTs)are registered with the relay node 7. Accordingly, users of mobiletelephones 3-1 and 3-2 can communicate with other users via the donorbase station 5-1 and the core network 8. Users of mobile telephones 3-3to 3-5 can communicate with other users via the relay node 7 and thedonor base station 5-1 and the core network 8. Users of the other mobiletelephones 4 can communicate with other users via the other base station5-2 and the core network 8.

To help ensure that the communication protocols used by the Relay MTs3-3, 3-4 and 3-5 and the Direct MTs remain consistent with one another,the relay node 7 effectively appears to be a base station to the RelayMTs 3-3, 3-4 and 3-5. Similarly, the base station 5 effectively appearsto be an MME to the relay node 7.

When an MME 9 experiences an overload situation, it randomly selects atleast one of the base stations 5 via which the mobile telephones 3, 4that it serves are registered (either directly via a base station 5 orindirectly via the relay node), to initiate a corresponding overloadprocedure. The overloaded MME 9 generates an ‘overload start’ messagecomprising an indication of the actions required in response to theoverload situation, and sends it to the selected base station(s) 5 torequest initiation of the corresponding overload procedure. When theoverloaded MME 9 is no longer overloaded it generates an ‘overload stop’message indicating that the requested overload procedure is no longerrequired and can therefore be discontinued.

In this embodiment, an MME 9 is able to request initiation of any ofthree different types of overload procedure by generating an overloadstart message including information identifying which of the three typesof procedure is required. The different overload procedures which can berequested in this manner comprise:

-   -   Rejection of all attempts to establish connections for        non-emergency mobile originated data transfer;    -   Rejection of all attempts to establish signalling connections;        and    -   Permitting attempts to establish connections for emergency        sessions and mobile terminated services only.

When the donor base station 5-1 has been selected by an MIME 9 toinitiate an overload procedure, and has received the associated overloadstart message, it initiates the requested overload procedure in respectof any Direct MTs 3-1 and 3-2 served by the overloaded MME 9. The donorbase station 5-1 also inserts information identifying the sending MME 9into the overload start message and forwards the modified overload startmessage (including the information identifying the sending MME 9) to therelay node 7. The relay node 7 reads the MME identification informationprovided with the overload start message and initiates the requestedoverload procedure only for the specific Relay MTs 3-3, 3-4 and 3-5served by the overloaded MME 9 represented by the MME identificationinformation read from the overload start message.

Accordingly, the relay node 7 is advantageously able to perform theactions required in response to the overload situation, selectively, forany Relay MTs 3-3 to 3-5 that the overloaded MME 9 serves, whilst anyRelay MTs 3-3 to 3-5 served by an MME 9 that is not overloaded, areunaffected.

When the donor base station 5-1 receives an overload stop message itterminates the requested overload procedure in respect of any Direct MTs3-1, 3-2 served by the MME 9 from which it has received the overloadstop message. The donor base station 5-1 also inserts informationidentifying the sending MME 9 into the overload stop message andforwards the modified overload stop message (including the informationidentifying the sending MME 9) to the relay node 7. The relay node 7reads the MME identification information sent with the overload stopmessage and discontinues the overload procedure only for any Relay MTs3-3 to 3-5 served by the MME 9 that sent the original overload stopmessage.

Accordingly, when a relay node 7 is carrying out parallel overloadprocedures in respect of a plurality of MMEs 9, the relay node 7 isadvantageously able to selectively discontinue the overload procedurefor mobile telephones 3-3 to 3-5 served by an MME 9 that ceases to beoverloaded, without affecting the other parallel overload procedure(s)for other MME(s) 9.

Base Station

FIG. 2 is a block diagram illustrating the main components in the donorbase station 5-1 shown in FIG. 1. As shown, the donor base station 5-1includes a transceiver circuit 31 which is operable to transmit signalsto, and to receive signals from, the Direct MTs 3-1 and 3-2 and therelay node 7 via at least one antenna 3. The donor base station 5-1 isalso operable to transmit signals to and to receive signals from theMMEs 9 of the core network 8 via a network interface 35. The operationof the transceiver circuit 31 is controlled by a controller 37 inaccordance with software stored in memory 39.

The software includes, among other things, an operating system 41, acommunications control module 43, an MME overload management module 45,an overload message handling module 47, and a Radio Resource Control(RRC) message handling module 49.

The communications control module 43 is operable to controlcommunication with the Direct MTs 3-1 and 3-2 and the relay node 7including, for example, allocation of resources to be used by thetransceiver circuit 31 in its communications with each of the Direct MTs3-1 and 3-2 and with the relay node 7. The communications control module43 also schedules the times for the transmission of downlink data to theDirect MTs 3-1 and 3-2 and the relay node 7.

The MME overload management module 45 initiates MME overload procedures,in accordance with the required overload actions indicated in associatedoverload start messages received from overloaded MMEs 9, in respect ofthe Direct MTs 3-1 and 3-2 served by the overloaded MMEs 9. Similarly,the MME overload management module 45 terminates MME overloadprocedures, in accordance with overload stop messages received from MMEs9 that cease to be overloaded, for the Direct MTs 3-1 and 3-2 served bythose MMEs 9.

The overload message handling module 47 receives overload messages (bothstart and stop messages) and extracts any information required by theMME overload management module 45 to initiate/terminate the MME overloadprocedures for the Direct MTs 3-1 and 3-2 (e.g. information identifyingthe type of overload actions required). The overload message handlingmodule 47 also identifies the MME 9 from which each overload message isreceived, inserts an identifier for the MME 9 which sent the overloadmessage into an MME identification information element provided in theoverload message, and forwards the modified message to the relay node 7.

The RRC message handling module 49 handles RRC messages received fromthe Direct MTs 3-1 and 3-2 by interpreting the RRC messages andresponding to the messages appropriately. When an overload procedure hasbeen initiated for an overloaded MME 9, the RRC message handling module49 responds to RRC connection establishment messages received fromDirect MTs 3-1 and 3-2, by determining whether or not the message hasbeen received from a mobile telephone 3 served by the overloaded MME 9.If a message has been received from a mobile telephone 3 served by theoverloaded MME 9, then the RRC message handling module 49 responds bygenerating a corresponding RRC message to reject or permit theconnection attempt in dependence on the type of overload procedurerequested by the overloaded MME 9 and the reason for the connectionestablishment message.

Relay Node

FIG. 3 schematically illustrates the main components of the relay node 7shown in FIG. 1. The relay node 7 may be a fixed communications nodelike a base station 5 or may itself be a mobile device. Indeed, somemobile telephones 3, 4 may provide relay services and therefore act asrelay nodes. As shown, the relay node 7 includes a transceiver circuit51 which is operable to transmit signals to, and to receive signalsfrom, Relay MTs 3-3 to 3-5 and the base station 5-1 via at least oneantenna 53. The operation of the transceiver circuit 51 is controlled bya controller 57 in accordance with software stored in memory 59.

The software includes, among other things, an operating system 61, acommunications control module 63, an MME overload management module 65,an overload message handling module 67, and a Radio Resource Control(RRC) message handling module 69.

The communications control module 63 is operable to controlcommunication with the Relay MTs 3-3 to 3-5 and the base station 5-1including, for example, allocation of resources to be used by thetransceiver circuit 51 in its communications with each of the Relay MTs3 and with the base station 5. The communications control module 63 alsoschedules the times for the transmission of appropriate sub-frames torelay associated data to the Relay MTs 3-3 to 3-5.

The overload message handling module 67 receives overload messages (bothstart and stop messages) forwarded by the donor base station 5-1 andextracts the MME identification information from the overload messages.The overload message handling module 67 also extracts any otherinformation required by the MME overload management module 65 toinitiate/terminate the MME overload procedures for the Relay MTs 3-3 to3-5 (e.g. information identifying the type of overload actionsrequired).

The MME overload management module 65 initiates MME overload procedures,in accordance with the required overload actions indicated in associatedoverload start messages received from the donor base station 5-1, inrespect of the Relay MTs 3-3 to 3-5 served by the MMEs 9 identified bythe MME identification information contained in the overload startmessages. Similarly, the MME overload management module 65 discontinuesany MME overload procedures, in accordance with overload stop messagesreceived from the donor base station 5-1, in respect of the Relay MTs3-3 to 3-5 served by the MMEs 9 identified by the MME identificationinformation contained in the overload stop message.

The RRC message handling module 69 handles RRC messages received fromthe Relay MTs 3-3 to 3-5 by interpreting the RRC messages and respondingto the messages appropriately. When an overload procedure has beeninitiated for an overloaded MME 9, the RRC message handling module 69responds to RRC connection establishment messages received from RelayMTs 3-3 to 3-5, by determining whether or not the message has beenreceived from a mobile telephone 3 served by the overloaded MME 9. If amessage has been received from a mobile telephone 3 served by theoverloaded MME 9, then the RRC message handling module 69 responds bygenerating a corresponding RRC message to reject or permit theconnection attempt in dependence on the type of overload procedurerequested by the overloaded MME 9 and the reason for the connectionestablishment message.

Overload Response Initiation Procedure

FIG. 4 illustrates, in general terms, a typical procedure followed bykey components of the telecommunication system 1 when an MME 9experiences an overload situation.

As seen in FIG. 4 when the MME 9 experiences an overload situation, itrandomly selects a subset of the base stations that it serves to performan overload procedure aimed at alleviating the overload situation (S10).The MME 9 also determines what type of overload procedure should beperformed by the selected base stations. In the example illustrated inFIG. 4, a donor base station 5 is selected to perform the overloadprocedure.

The MME 9 generates an overload start message (S1 AP: Overload Start)that includes, amongst other information, an Overload Action informationelement which is set by the MME 9 to indicate the type of overloadprocedure required. This message is then sent to the selected basestations 5 including the donor base station 5-1 (S12). The type ofoverload procedure requested by the base station 5 is represented bydata in the Overload Action IE of the overload start message. Asdescribed previously, the different types of overload procedure that abase station 5 may perform comprise:

-   -   Rejection of attempts to establish connections for non-emergency        mobile originated data transfer;    -   Rejection of attempts to establish signalling connections; or    -   Permitting attempts to establish connections for emergency        sessions and mobile terminated services.

On receipt of the overload start message the donor base station 5-1determines what type of overload procedure is required, from theOverload Action IE, and initiates the corresponding overload procedureaccordingly for the Direct MTs 3-1 and 3-2 which the overloaded MME 9serves (S14). The donor base station 5-1 also modifies the overloadstart message by inserting information identifying the MME source of theoverload start message into an MME ID information element in theoverload start message and forwards the modified message to the relaynode 7 (S16).

On receipt of the overload start message from the donor base station5-1, the relay node 7 can determine what type of overload procedure isrequired, from the Overload Action IE and can determine which MME 9 theoverload start message relates to from the MME ID IE. The relay node 7initiates the corresponding overload procedure selectively for the RelayMTs 3-3 to 3-5 that the overloaded MME 9 serves (S18).

Overload Response Termination Procedure

FIG. 5 illustrates, in general terms, a typical procedure followed bykey components of the telecommunication system 1 when an MME 9, that waspreviously experiencing an overload situation, ceases to be overloadedand requires any overload procedures being performed by the basestations it serves to be discontinued.

As seen in FIG. 5, when the MME 9 determines that the overload situationhas terminated (S20), it generates an overload stop message (S1 AP:Overload Stop) and sends it to the base stations 5 performing overloadprocedures, including the donor base station 5-1 (S22).

On receipt of the overload stop message the donor base station 5-1 stopsthe corresponding overload procedure accordingly for the Direct MTs 3-1and 3-2 which the overloaded MME 9 serves (S24). The donor base station5-1 also modifies the overload stop message by inserting informationidentifying the MME source of the overload stop message into an MME IDinformation element and forwards the modified message to the relay node7 (S26).

On receipt of the overload stop message from the donor base station 5-1,the relay node 7 determines which MME 9 the overload stop messagerelates to from the MME ID IE. The relay node 7 terminates thecorresponding overload procedure being performed for the Relay MTs 3-3to 3-5 which the MME 9 represented by the MME ID IE serves (S28).

Overload Procedure

FIG. 6 illustrates, in general terms, an exemplary overload procedurefollowed by a relay node 7 of the telecommunication system 1 when aspecific overloaded MME 9-1 (MME1 in the illustrated example) hasrequested the overload procedure.

A Relay MT 3-3 served by the overloaded MME 9-1 attempts to establish aconnection by generating an RRC connection request and sending it to therelay node 7 (S32, S38). The RRC connection request includes, amongstother information, an establishment cause information element foridentifying what the attempted connection establishment is for. Theestablishment cause information element may, for example, be set to anyof the following:

-   -   ‘mo-data’—to indicate that the attempted connection        establishment is for a mobile originated (mo) data transfer;    -   ‘mo-signalling’—to indicate that the attempted connection        establishment is for mobile originated (mo) signalling;    -   ‘mt-Access’—to indicate that the attempted connection        establishment is for a mobile terminated (mt) service;    -   ‘emergency’—to indicate that the attempted connection        establishment is for an emergency session; or    -   lighPriorityAccess'—to indicate that the attempted connection        establishment is for highPriorityAccess.

The relay node 7 establishes that the Relay MT 3-3 requesting theconnection is served by the overloaded MME 9-1 for which the overloadprocedure has been initiated. If MT 3-3 requesting the connection isserved by the overloaded MME 9-1, then the relay node 7 reads theestablishment cause IE and determines whether it indicates that the RRCconnection request should be rejected or allowed (S34, S39). The relaynode 7 then responds to the connection request by generating and sendingan RRC connection reject message (if the connection is to berejected—S36) or an RRC connection accept message (if the connection isto be accepted—S40).

The different overload reject/accept actions for the different overloadprocedures are summarised below:

-   -   Rejection of attempts to establish connections for non-emergency        mobile originated data transfer→reject connection attempt if        establishment cause is ‘mo-data’, otherwise permit;    -   Rejection of attempts to establish signalling connections→reject        connection attempt if establishment cause is ‘mo-data’ or        ‘mo-signalling’, otherwise permit; or    -   Permitting attempts to establish connections for emergency        sessions and mobile terminated services→permit connection        attempt if establishment cause is ‘emergency’ or ‘mt-access’,        otherwise reject.

Thus, only signalling traffic corresponding to permitted RRC connectionsis sent to the overloaded MME 9-1.

When rejecting the RRC connection request for overload reasons the relaynode 7 indicates, to the affected Relay MT, the cause of the rejectionand an appropriate timer value, for limiting further RRC connectionrequests within a specified time period.

If the relay node 7 establishes that the Relay MT 3-5 requesting theconnection is served by an MME 9-2 for which an overload procedure hasnot been initiated then, the relay node 7 then accepts (or rejects) theconnection request in accordance with LTE standard procedures withoutregard to the overload procedure initiated for the overloaded MME 9-1.

Accordingly, the relay node 7 is advantageously able to perform theactions required in response to the overload situation being experiencedby MME 9-1 for any Relay MT 3-3 served by MME 9-1 without affectingacceptance or rejection of connection requests received from Relay MT3-5 that is being served by MME 9-2, which is not overloaded.

Modifications and Alternatives

A detailed embodiment has been described above. As those skilled in theart will appreciate, a number of modifications and alternatives can bemade to the above embodiment whilst still benefiting from the inventionsembodied therein.

In an alternative embodiment, for example, the donor base station doesnot forward the overload start (or overload stop) message to the relaynode and, accordingly does not inform the relay node that a particularMME has become overloaded. Accordingly, in this embodiment, the relaynode 7 will accept an RRC connection request from the Relay MT withoutregard to any overload procedure requested by the MME 9 which serves theRelay MT. However, the donor base station 5 of this embodiment isoperable to detect attempts by a Relay MT being served by an overloadedMME 9 to become active (e.g. to establish communications with anothermobile telephone or communications device in the system), and to preventor allow the attempt in dependence on the reason for the attempt,thereby effectively implementing the requested overload procedure.

In this embodiment, as the skilled person will appreciate, the donorbase station 5 is able to identify the reason for the attempt by theRelay MT (served by the overloaded MME) to become active from an RRCestablishment cause information element contained in a standard S1 APInitial UE message sent to it by the relay node. The donor base stationthen determines if the attempt by the Relay MT to become active shouldbe allowed based on the establishment cause as described previously. Ifthe donor base station decides that the attempt should not be allowed(based on the indication given in the overload action IE of the overloadstart message) it generates a reply to the S1 AP Initial UE message toterminate the connection with the Relay MT (even though it was acceptedby the relay node).

The S1AP Initial UE message is a class 2 procedure and currently afailure message for replying to the S1 AP Initial UE message has notbeen specified.

A possible reply message that the donor base station may use, to preventthe Relay MT entering the active state comprises an error indicationmessage, receipt of which causes the relay node 7 to release theconnection with the affected Relay MT using RRC Connection Release.

Whilst an error indication may be used to allow the donor base station 5selectively to prevent activation of Relay MTs for overload reasons, ithas the following possible issues:

-   -   As mentioned above, the RRC connection will already be        established by the time the relay node sends the Initial UE        message. Thus, this solution does not conform to the principles        of the MME overload functionality which, ideally, involves        outright rejection of RRC Connection with an appropriate wait        timer. Moreover, RRC Connection Release does not contain timer        information, to allow the period within which further connection        attempts should not be made to be set.    -   Ideally, error indication should only be used to report detected        errors in an incoming message rather than to support the        overload functionality.

Accordingly, in an embodiment using an error indication in this way, thedonor base station 5 also indicates an associated cause to the relaynode 7. On determining that the cause of the error indication isoverload related, the relay node 7 can thus set an appropriate timervalue to limit further RRC connection requests for an associated timeperiod.

Another possible reply message that the donor base station 5 may use, toprevent the Relay MT entering the active state comprises a UE ContextRelease message, receipt of which causes the relay node 7 to release theconnection with the affected Relay MT using RRC Connection Release.

Whilst a UE context release may be used to allow the donor base station5 to selectively prevent activation of Relay MTs for overload reasons,it has the following possible issues:

-   -   As with use of an ‘error indication’, the RRC connection will be        already established by the time the relay node 7 sends the        Initial UE message. Thus, this solution does not conform to the        principles of the MME overload functionality which, ideally,        involves outright rejection of RRC Connection with an        appropriate wait timer. Moreover, RRC Connection Release does        not contain timer information, to allow the period within which        further connection attempts should not be made to be set.    -   The purpose of the UE Context Release procedure is to enable an        MME to order the release of a UE-associated logical connection        due to various reasons. However, the UE-associated logical        connection is not fully established at the time the donor base        station 5 receives the Initial UE message.

In the above embodiments, a mobile telephone based telecommunicationssystem was described. As those skilled in the art will appreciate, thesignalling techniques described in the present application can beemployed in other communications system. Other communications nodes ordevices may include user devices such as, for example, personal digitalassistants, laptop computers, web browsers, etc. As those skilled in theart will appreciate, it is not essential that the above described relaysystem be used for mobile communications devices. The system can be usedto extend the coverage of base stations in a network having one or morefixed computing devices as well as or instead of the mobilecommunicating devices.

In the above description, the donor base station 5-1 and the relay node7 are described, for ease of understanding, as having a number ofdiscrete modules. Whilst these modules may be provided in this way forcertain applications, for example where an existing system has beenmodified to implement the invention, in other applications, for examplein systems designed with the inventive features in mind from the outset,these modules may be built into the overall operating system or code andso these modules may not be discernible as discrete entities.

Where a plurality of MMEs become (or cease to be) overloaded, a list ofMME identifiers may be provided in the associated overload start (oroverload stop) message.

Whilst modifying the overload messages to include informationidentifying the source of the overload message and forwarding themodified message to the relay node is advantageous in terms ofsimplicity, ease of implementation and minimising the number of messagesrequired, the information identifying the source of the overload messagemay be sent in any of a number of different ways. The information may,for example, be sent in a separate message either before or after theoverload message is forwarded. Moreover, instead of modifying andforwarding a received overload message, a completely new message may begenerated which includes the information identifying the source of theoverload message and any information required to initiate/terminate theassociated overload procedures.

While it is beneficial, in terms of ease of implementation, for the basestation to insert an MME's identity into the overload start or stopmessage, it will be appreciated that an MME sending overload message mayinsert its own identity into the overload message prior to sending it tothe selected base stations.

In the embodiments described above, the relay station and the basestation will each include transceiver circuitry. Typically thiscircuitry will be formed by dedicated hardware circuits. However, insome embodiments, part of the transceiver circuitry may be implementedas software run by the corresponding controller.

In the above embodiments, a number of software modules were described.As those skilled in the art will appreciate, the software modules may beprovided in compiled or un-compiled form and may be supplied to the basestation or the relay station as a signal over a computer network, or ona recording medium. Further, the functionality performed by part or allof this software may be performed using one or more dedicated hardwarecircuits. Various other modifications will be apparent to those skilledin the art and will not be described in further detail here.

The following is a detailed description of the way in which the presentinventions may be implemented in the currently proposed 3GPP standard.Whilst various features are described as being essential or necessary,this may only be the case for the proposed 3GPP standard, for exampledue to other requirements imposed by the standard. These statementsshould not, therefore, be construed as limiting the present invention inany way.

1 Introduction

The objective of the paper is to analyse how the MME Overload functionworks in an RN deployment scenario. The paper highlights related issuesand analyse different solution (e.g. MME Overloading terminating to DeNBor RN). Also the paper highlights pros and cons of the differentsolution and it propose a way forward accordingly.

2 Background MME Overload

The MME Overload procedure was introduced to control overload situationin MIME. When MME experience an overload situation, it will randomlychoose some eNBs towards which the MME Overload procedure is sent. TheMME will indicate to the eNB which action should eNB undertake uponoverload indication.

Depending on the Overload Action IE in SLAP overload start, eNB shall

-   -   “reject all RRC connection establishments for non-emergency        mobile originated data transfer” (i.e. reject traffic        corresponding to RRC cause “mo-data”), or    -   “reject all RRC connection establishments for signalling” (i.e.        reject traffic corresponding to RRC cause “mo-data” and        “mo-signalling”), or    -   “only permit RRC connection establishments for emergency        sessions and mobile terminated services” (i.e. only permit        traffic corresponding to RRC cause “emergency” and “mt-Access”).

When rejecting an RRC connection request for overload reasons the eNBindicates to the LIE an appropriate timer value that limits further RRCconnection requests for a while [1].

MME Pool and RN Deployment

According to the agreed Relay architecture, DeNB will be seen as an MMEfrom RN view. As a consequence RN will not have any knowledge on whichMME has triggered a certain non UE associated procedure.

3 Discussion

In FIG. 7, RN is serving UE1 and UE2: UE1 is being served by MME1 andUE2 is being served by MME2. Assuming MME1 will get overloaded, it willsend an Overload message to DeNB. DeNB need to take action uponreception of S1AP Overload start e.g. reject the RRC connection withestablishment cause as indicated by the MME.

During the overload situation, the affected UEs being served by the RNneed to be handled as well. Different solutions are discussed below:

-   -   DeNB does not inform the RN that a certain MME is overloaded.        This means that it is up to the DeNB to reject any UE attempt to        go to active state if the UE is being served by the overloaded        MME and the cause for going to active is in line with the        indication given by the overloaded MME. The DeNB will be aware        about the reason of the RRC connection establishment request        from the RRC Establishment cause IE in S1AP Initial UE message.        However, the S1AP Initial UE message is a class 2 procedure and        a failure message is not specified. Two possible reply are        considered below.        -   Error Indication            -   Problem in using Error indication                -   The RRC connection will be already established by                    the time the RN sends the Initial UE message. Thus                    this solution is not in line with the principles of                    the MME Overload functionality which foresee the                    rejection of RRC Connection with an appropriate wait                    timer. RRC Connection Release does not contain the                    wait timer information.                -   Error indication should only be used when to report                    detected errors in one incoming message and not as                    support to CP functionality e.g. Overload function                -   If it will be decided to go for Error Indication,                    text procedure would be needed to clarify that the                    DeNB need to indicate appropriate cause so that RN                    can an appropriate timer value that limits further                    RRC connection requests for a while.        -   UE Context Release            -   Problems in using UE Context Release                -   The RRC connection will be already established by                    the time the RN send the Initial UE message (same                    problem as for the error indication). Thus this                    solution is not in line with the principles of the                    MME Overload functionality which foresee the                    rejection of RRC Connection with an appropriate wait                    timer. RRC Connection Release does not contain the                    wait timer information.                -   The purpose of the UE Context Release procedure is                    to enable the MME to order the release of the                    UE-associated logical connection due to various                    reasons. However the UE associated logical                    connection is not yet established at the time DeNB                    receives the Initial Ue message [2].    -   DeNB forward the Overload message to the RN        -   Possible problem            -   the RN does not know which MME is currently overloaded                and it may end up rejecting unnecessarily all attempted                RRC Connections establishment e.g. it may end up reject                RRC Connection establishment from both UE1 and UE2 when                actually just MME2 is overloaded (FIG. 2).        -   Solution            -   In order to overcome this issue, the DeNB should                indicate to the RN which MME is overloaded when                forwarding the S1AP overload start and Overload stop.                This information may also be useful in case e.g. two MME                gets overloaded (via allowing parallel procedure in Uu                or list of MME id in the S1AP Overload messages)

Proposals:

-   -   It is proposed that the overload messages are forwarded by the        DeNB to the RN    -   It is proposed to add a new optional IE In S1AP overload related        messages so that RN is able to identify the affected RRC        connection request.

4 Conclusion

This paper has analysed issues related to the MME Overload function incase of RN deployment. It is proposed to discuss the paper and agree onthe proposal in chapter 3.

5 Reference

-   [1]23.401: General Packet Radio Service (GPRS) enhancements for    Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access-   [2] 36.401: Evolved Universal Terrestrial Radio Access Network    (E-UTRAN); Architecture description

This application is based upon and claims the benefit of priority fromUnited Kingdom patent application No. 1013559.8, filed on Aug. 12, 2010,the disclosure of which is incorporated herein in its entirety byreference.

1. A relay node for a communication system comprising a plurality ofmobility management entities (MMEs), the relay node comprising: meansfor associating a plurality of mobile devices with the relay nodewherein each mobile device is served by an MME of said plurality ofMMEs; means for receiving, from a base station, a request that anoverload action be performed and identity information representing anidentity of an MME that requested the overload action; and means forselectively performing said requested overload action in respect of themobile devices associated with the relay node in dependence on theidentity information representing an identity of an MME that requestedthe overload action.
 2. A relay node as claimed in claim 1 wherein therequested overload action comprises initiating an overload procedure. 3.A relay node as claimed in claim 2 wherein the overload procedurecomprises: receiving a request to establish a connection from a mobiletelephone associated with the relay node; determining whether or not themobile telephone that made the request is served by the MME having theidentity represented by the identity information; generating a messagefor rejecting said request to establish a connection or generating amessage for accepting said request to establish a connection independence on whether or not the mobile telephone that made the requestis served by the MME having the identity represented by the identityinformation; and transmitting said generated message to the mobiletelephone that made the request to establish a connection.
 4. A relaynode as claimed in claim 3 wherein the overload procedure initiated bythe performing means further comprises determining a reason for therequested connection establishment request, and wherein said generationstep further comprises generating said message for rejecting saidrequest to establish a connection or generating said message foraccepting said request to establish a connection in dependence on saiddetermined reason for the requested connection.
 5. A relay node asclaimed in claim 1 wherein the requested overload action comprisesterminating an overload procedure.
 6. A base station for a communicationsystem comprising a plurality of mobility management entities (MMEs),the base station comprising: means for receiving, from an MME, a requestthat an overload action be performed; means for generating identityinformation representing an identity of the MME that requested theoverload action; and means for transmitting, to a relay node, therequest that the overload action be performed and the informationrepresenting an identity of the MME that requested the overload action.7. A base station as claimed in claim 6 further comprising means forinserting said information representing an identity of the MME thatrequested the overload action into said request that the overload actionbe performed, whereby said information representing an identity of theMME that requested the overload action is transmitted as part of saidrequest that the overload action be performed.
 8. A base station asclaimed in claim 6 wherein said request that the overload action beperformed comprises a request for an overload procedure to be initiated.9. A base station as claimed in claim 6 wherein said request that theoverload action be performed comprises a request for an overloadprocedure to be terminated.
 10. A method performed by a relay node in acommunication system comprising a plurality of mobility managemententities (MMEs), the method comprising: associating a plurality ofmobile devices with the relay node wherein each mobile device is servedby an MME of said plurality of MMEs; receiving, from a base station, arequest that an overload action be performed and identity informationrepresenting an identity of an MME that requested the overload action;and selectively performing said requested overload action in respect ofthe mobile devices associated with the relay node in dependence on theidentity information representing an identity of an MME that requestedthe overload action.
 11. A method performed by a base station in acommunication system comprising a plurality of mobility managemententities (MMEs), the method comprising: receiving, from an MME, arequest that an overload action be performed; generating identityinformation representing an identity of the MME that requested theoverload action; and transmitting, to a relay node, the request that theoverload action be performed and the information representing anidentity of the MME that requested the overload action.
 12. Acommunications device for a communication system that comprises aplurality of mobility management entities (MMEs), a base station, arelay node, and a plurality of mobile devices associated with the relaynode the communications device comprising: means for receiving a requestthat an overload action be performed for at least one of said pluralityof MMEs and identity information representing an identity of an MME thatrequested the overload action; and means for selectively performing saidrequested overload action in respect of the mobile devices associatedwith the relay node in dependence on the identity informationrepresenting an identity of an MME that requested the overload action.13. A communications device as claimed in claim 12 wherein saidcommunications device is said relay node.
 14. A communications device asclaimed in claim 13 wherein the requested overload action comprisesinitiating an overload procedure, and wherein said overload procedurecomprises: receiving a request to establish a connection from a mobiletelephone associated with the relay node; determining whether or not themobile telephone that made the request is served by the MME for whichthe overload action was requested; generating a message for rejectingsaid request to establish a connection or generating a message foraccepting said request to establish a connection in dependence onwhether or not the mobile telephone that made the request is served bythe MME for which the overload action was requested; and transmittingsaid generated message to the mobile telephone that made the request toestablish a connection.
 15. A communications device as claimed in claim12 wherein said communications device is said base station.
 16. Acommunications device as claimed in claim 15 wherein the requestedoverload action comprises initiating an overload procedure, and whereinsaid overload procedure comprises: receiving an indication that aconnection has been established between a mobile telephone associatedwith the relay node and the relay node; determining whether or not themobile telephone for which the connection is established is served bythe MME for which the overload action has been requested; selectivelygenerating a message for terminating said established connection independence on whether or not the mobile telephone for which theconnection is established is served by the MME for which the overloadaction has been requested; and transmitting any message so generated tothe relay node.
 17. A communications device as claimed in claim 16wherein the message for terminating said established connectioncomprises an error indication.
 18. A communications device as claimed inclaim 16 wherein the message for terminating said established connectioncomprises a message for releasing a context of the mobile telephone. 19.A method performed by a communications device in a communication systemthat comprises a plurality of mobility management entities (MMEs), abase station, a relay node, and a plurality of mobile devices associatedwith the relay node, the method comprising: receiving a request that anoverload action be performed for at least one of said plurality of MMEsand identity information representing an identity of an MME thatrequested the overload action; and selectively performing said requestedoverload action in respect of the mobile devices associated with therelay node in dependence on the identity information representing anidentity of an MME that requested the overload action.
 20. Acommunication node for a communication system comprising a plurality ofmobility management entities (MMEs), the communication node comprising:means for associating a plurality of mobile devices with thecommunication node wherein each mobile device is served by an MME ofsaid plurality of MMEs; means for receiving, from a furthercommunication node, a request that an overload action be performed andidentity information representing an identity of an MME that requestedthe overload action; and means for selectively performing said requestedoverload action in respect of the mobile devices associated with saidcommunication node in dependence on the identity informationrepresenting an identity of an MME that requested the overload action.21. A communication node for a communication system comprising aplurality of mobility management entities (MMEs), the base stationcomprising: means for receiving, from an MME, a request that an overloadaction be performed; means for generating identity informationrepresenting an identity of the MME that requested the overload action;and means for transmitting, to a further communication node, the requestthat the overload action be performed and the information representingan identity of the MME that requested the overload action.
 22. A methodperformed by a communication node in a communication system comprising aplurality of mobility management entities (MMEs), the method comprising:associating a plurality of mobile devices with the communication nodewherein each mobile device is served by an MME of said plurality ofMMEs; receiving, from a further communication node, a request that anoverload action be performed and identity information representing anidentity of an MME that requested the overload action; and selectivelyperforming said requested overload action in respect of the mobiledevices associated with said communication node in dependence on theidentity information representing an identity of an MME that requestedthe overload action.
 23. A method performed by a communication node in acommunication system comprising a plurality of mobility managemententities (MMEs), the method comprising: receiving, from an MME, arequest that an overload action be performed; generating identityinformation representing an identity of the MME that requested theoverload action; and transmitting, to a further communication node, therequest that the overload action be performed and the informationrepresenting an identity of the MME that requested the overload action.24. A computer program product comprising computer implementableinstructions for causing a programmable computer device to becomeconfigured as the relay node of claim
 1. 25. A computer program productcomprising computer implementable instructions for causing aprogrammable computer device to become configured as the base station ofclaim
 6. 26. A computer program product comprising computerimplementable instructions for causing a programmable computer device tobecome configured as the communications device of claim
 12. 27. Acomputer program product comprising computer implementable instructionsfor causing a programmable computer device to become configured as acommunication node according to claim
 20. 28. A computer program productcomprising computer implementable instructions for causing aprogrammable computer device to become configured as a communicationnode according to claim 21.